Fungo fun

This week's PostCards question:

Dickson presents five theories on the etymology of the word “fungo", as in “fungo bat" or “catching fungoes". Define at least one of the following series and for a gold sticker identify which one was advocated by a professor at the University of Missouri-Rolla. The etymological explanations:

1. The ‘Fun/Go’ Theory
2. The Fungible Theory
3. The Fungus Theory
4. The Fangen Theory
5. The Fung Theory

Write postcards@post-dispatch.com with your answers.